Scientific pitch notation
Scientific pitch notation (or SPN, also known as American Standard Pitch Notation (ASPN) and International Pitch Notation (IPN)) is a method of specifying musical pitch by combining a musical note name (with accidental if needed) and a number identifying the pitch's octave.
Although scientific pitch notation (SPN) was originally designed as a companion to "scientific pitch" (see below), the two are not synonymous, and should not be confused. Scientific pitch is a pitch standard—a system which defines the specific frequencies of particular pitches (see below). SPN concerns only how pitch names are notated, that is, how they are designated in printed and written text, and does not inherently specify actual frequencies. Thus the use of SPN to distinguish octaves does not depend on the pitch standard used.
The octave number increases by 1 upon an ascension from B to C (and not from G to A, as one might expect). Thus "A4" refers to the first A above C4 (middle C). In describing musical pitches, enharmonic spellings can give rise to anomalies where C♭4 is a lower frequency than B♯3; such paradoxes do not arise in a scientific context.
Scientific pitch notation is often used to specify the range of an instrument. It provides an unambiguous means of identifying a note in terms of textual notation rather than frequency, while at the same time avoiding the transposition conventions that are used in writing the music for instruments such as the clarinet and guitar. It is also easily translated into staff notation, as needed.
Other traditional octave naming systems—where for example C0 is written as ′′C, or C, or CCC in Helmholtz pitch notation, or referred to as subcontra C, and where C4 is written as c′ or one-lined C—applies to the written notes that may or may not be transposed. For example, a d′ played on a B♭ trumpet is actually a C4 in scientific pitch notation.
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